A beaker has 20 litres of a 60% acid solution. 5 litres are removed and replaced with water. What is the acid concentration now?

Explanation

1. 01

   Acid at first = $0.60 \times 20 = 12$ litres.

2. 02

   Removing 5 of 20 litres removes $\frac{1}{4}$ of the acid, leaving $12 \times \frac{3}{4} = 9$ litres.

3. 03

   The volume is back to 20 litres, so concentration = $\frac{9}{20} = 45\%$.